Field of Invention
Various embodiments of the present disclosure relate to a light generation and absorption technology, and more particularly, to a continuous frequency variable and pulsed broadband photomixer technology for generating terahertz (THz) waves.
Description of Related Art
Terahertz (THz) waves are defined as waves in the region of 0.1 to 10 THz, where 1 THz is 1012 Hz, in an electromagnetic spectrum band. In particular, there are gyro and resonant frequencies of various molecules in the 0.1 to 3 THz region. Molecular fingerprints are obtained by using non-destructive, non-opening, and non-contact methods by utilizing the THz waves and thus it is possible to provide an advanced concept future core technology in medical treatment, medicine, agri-food, environment measurement, biology, communication, non-destructive inspection, and advanced material evaluation. Thus, intense competition is progressing in order to develop a related core technology.
The THz waves have little influence on a human body because they have very low energy of several meV. Accordingly, a THz wave processing technology is emerging as one of core technologies for realizing a human-oriented ubiquitous society, and it is expected that demands therefor sharply increase. Although a technology satisfying real time, portable, low-cost, and broadband issues at the same time has not yet been developed, various technologies on THz spectroscopy and imaging field utilization are being proposed due to persistent enhancement in technology.
The THz imaging field essentially employs a high output power and high sensitivity array type detector. On the other hand, since a broadband THz wave source is a core technology in the THz spectroscopy field, it is based on an optical technology. Hereinafter, a typical broadband THz system will be described with reference to drawings.
FIG. 1 is a view illustrating a configuration of a typical pulsed THz time domain spectroscopy (TDS) system.
Referring to FIG. 1, a typical broadband THz system is configured with a femtosecond-level high output pulse laser and a photoconductive antenna (PCA), and, by using a femtosecond-level ultra-short pulse laser, irradiates femtosecond-level ultra-short pulse laser beams on a semiconductor having an ultra high response speed to generate THz waves. Here, the femtosecond-level ultra-short pulse laser may be a Ti: sapphire laser, and the PCA, which is a THz wave generator due to femtosecond light excitation, may be configured with a super-high frequency photoelectric converter.
According to this structure, in the broadband THz system, 800 nm, which is a central oscillation wavelength of the commercialized Ti: sapphire laser, is absorbed and low-temperature-grown GaAs having a very short carrier lifetime is utilized as an active material of the PCA. In a configuration of the THz spectroscopy system, it is essential to adopt a material for efficiently absorbing excitation light or having a femtosecond-level carrier lifetime necessary for a broadband characteristic.
The broadband THz spectroscopy system is a first commercialized system because it is relatively easy to realize a high signal-to-noise ratio (SNR) and broadband characteristics. However, since a THz-TDS system is configured with a sophisticated and complicated optical system including a femtosecond-level ultra-short pulse laser and an optical delay, it is expensive and has a large size. In addition, at the time of measuring a time domain signal, it is difficult to measure in real time due to an optical delay time and a Fast Fourier Transform (FFT) signal processing time of the measured time domain signal.
FIG. 2 is a view illustrating a configuration of a continuous wave lasing THz-frequency domain spectroscopy (FDS) system.
Referring to FIG. 2, the THz-TDS system uses beating generated by two very stable and high output power wavelengths instead of the femtosecond-level ultra-short pulse laser beams as excitation laser beams. Except for a light source, a THz wave generating manner is similar to that of the THz-TDS system described in relation to FIG. 1.
For the PCA that is a super-high frequency photoelectric converter for the THz-TDS, it is possible to easily generate broadband THz waves by using a several micrometer-sized quadrilateral light excitation region and a very simple dipole antenna due to a high peak value of the ultra-short pulse laser beam. On the contrary, in the THz-FDS system of FIG. 2, since a THz wave having a frequency corresponding to the difference between two wavelengths is generated, a term “photomixer” is used instead of the term “PCA”.
The pulsed THz-TDS system may provide high frequency resolution according to the continuous wave manner. In addition, the pulsed THz-TDS system uses two independent high output semiconductor lasers and accordingly enables low-price, broadband, and micro-sized system development. Therefore, since it is possible to develop a THz spectroscopy system as a site application type, many organizations competitively develop related technologies. However, the continuous wave manner does not show concrete and substantial system application cases due to poor photoelectric conversion efficiency thereof.